A team from the Abant Izzet Baysal University of Duzce in Turkey have issued the warning after looking at 19 cases of honey-related food poisoning.

The patients all required emergency care after eating untreated honey, known as "mad" honey, which contained a toxin derived from certain plants.

The British honey industry said produce sold commercially in the UK will have been processed to remove impurities.

The people in Turkey, aged between 22 and 61 and from the Black Sea coastal area of Turkey, had all eaten between 30 and 180 grams of the "mad" honey before falling ill in 2002.

Their symptoms included nausea, vomiting, low blood pressure, slowed heart rate and fainting. Four of them also sustained head injuries from failing.

Toxin

Fifteen of the patients had previously been diagnosed with a duodenal ulcer. The honey is sometimes used as an alternative to medicine to treat stomach and bowel problems, but it contains a toxin called grayanotoxin I.

The toxin is found in the leaves and flowers of rhododendrons and azaleas and is extracted by bees before finding its way into honey.

But the report said all patients are susceptible to the toxin and as little as a teaspoon of natural honey may be sufficient to cause poisoning.

Dr Hakan Ozhan, of the university's cardiology clinic, said people should be careful when buying natural honey, especially direct form beekeepers.

"There is growing demand for natural honey, people prefer natural things theses days but that does not necessarily mean it is good for you.

"The industrialisation process takes these toxins out of the honey.

"If people are going to buy natural honey they should look at the ingredients and see if it is produced in a country where rhododendrons grow."

Industry

The UK honey market is worth £40m a year and more than 25,000 tonnes of honey are consumed each year in the UK.

More than 90% of honey is imported from abroad.

It is imported in a raw "unprocessed" form before undergoing a heating process to take out impurities.

The Food Standards Agency said there was no need for British consumers to be concerned.

A spokeswoman said: "There have been no reported cases of natural honey poisoning in the UK.

"Hive bees get little nectar from ericaceous plants like rhododendrons in the UK, and the UK imports very little honey from Turkey."

And the Honey Association, a trade body for honey importers, said: "All honeys imported into the UK are rigorously checked both before and after shipping to ensure they meet all EU regulations.

"The Honey Association also undertakes regular surveillance testing to check compliance with these, as does the government."